The only way I can see to do it would be 3M tape and some tweeters with angled mounts. Mount them near the seam between the 2 panels and use that seam for the cable.
Out of curiosity whats the reason for wanting them? Do you have a lot of people in the back? There's zero point in doing it from a drivers perspective as high frequencies drop too quickly, unless you mount them on the B pillars, but then you're better off with them in front of you. Music isn't designed, or recorded from a surround perspective, it's all centre stage, left/right channels, so you'd only be repeating what the fronts are playing. Then there's the whole issue of phase cancellation in a small cabin.
